Periodic intensification of routine immunization (PIRI): modeling a novel strategy to supplement routine and pulsed measles vaccination

BackgroundRoutine immunization (RI) is widely used to increase population immunity against measles. In low-resource settings, achieving immunity goals using RI alone has proved challenging and supplemental immunization activities (SIAs), large community-based vaccination campaigns conducted every few years, have been used to close immunity gaps. Although effective at covering the population unreached by RI and boosting the population immunity, SIAs are labor-intensive and expensive, allowing for accumulation of susceptible in-between campaigns. Periodic intensification of routine immunization (PIRI) has been used in settings with high measles incidence as another supplement to the existing immunization framework. PIRI is built on the existing RI infrastructure and often more frequently than SIAs. However, the effects of PIRI have not been quantitatively evaluated against SIAs. MethodsWe developed a stochastic age-structured SIR model parameterized by measles dynamics to simulate RI, RI plus SIAs, and RI plus PIRI vaccination strategies to quantify their effects on measles control. We define the sufficient PIRI rate as the immunization rate during a PIRI, relative to RI, that achieves equal or fewer cases than a corresponding RI plus SIAs strategy. ResultsWe found a U-shaped relation between the sufficient PIRI rate and RI coverage levels. When RI coverage levels are low or high, PIRI cannot reduce cases below the level achieved by RI plus SIA. But at intermediate RI coverage levels, the sufficient PIRI rate is low; approximated 4-times the RI rate, and PIRI achieves fewer cases than SIAs. We also found that in scenarios of increasing RI coverage, maintaining PIRIs even after this intermediate regime results in a negligible increase in cases and lower annual variability. ConclusionPIRI has the potential to meet or exceed the performance of an SIAs-based strategy in settings with intermediate levels of RI coverage or settings with high and continuously increasing RI coverage.